He says sulfur-laden mine waste has built up in and around the Fly River and if the river's floodplains dry out, this could trigger a mass poisoning of animals and plants.

Dr Campbell, who has also previously served with the Mekong River Commission, says if the sediments dry out and are exposed to oxygen, the sulfur will oxidise and create sulfuric acid, which dissolves toxic metals and creates a long-term toxicity.

He says his reading of company data suggests around 150 square kilometres of the Fly River floodplains, downstream from the Ok Tedi mine could be potentially affected by acid mine drainage.

Company response

Ok Tedi Mining Limited says they are aware of the problem of acid mine drainage and a few pockets (the largest of which extend up to a few hundred square metres) of oxidation had already occurred.

But the company disagrees with Dr Campbell on the extent of the area that could be affected by acidity.

He says the river floodplains are very unlikely to dry out and flooding is expected to increase over the next 200 years.

Storage pits

As part of the solution, the Ok Tedi mine is in the process of storing extracted sulfur from the mine waste in a pit on the floodplain of the Ok Tedi river, just upstream from its junction with the Fly River.

But Dr Campbell says this could compound the problem if the river changes its course and cuts into the pit, releasing a pulse of sulfur into the river.

Mr Breen says the new pit for storing sulfur extracted from mine waste has been independently assessed as safe by an expert review team.

He adds that experts have also concluded that "for between 100 to 1,000 years, there is minimal risk of the Ok Tedi channel migrating towards the storage pits."

"This company spends more than $1.5 million each year on world-leading environmental consultants, including a number of Australian and other universities and the CSIRO," Mr Breen said.

"Dr Campbell's view of some of the scientific findings digressed somewhat from this collective viewpoint."

Support for concerns

Environmental engineer Dr Gavin Mudd of Monash University, who specialises in mine waste management, says a number of research papers have "played down" the acid mine drainage problem.

Dr Mudd, who has worked for mining companies as well as Aboriginal communities and has been following the Ok Tedi issue since 1994, agrees with Dr Campbell's estimates of the potential area affected.

"I think that's a much more realistic assessment of the nature of the problem at Ok Tedi," Dr Mudd said.

He says there is scientific controversy over how the mine waste will behave in the river, and whether calcium carbonate - either naturally occurring or deposited with the waste - can prevent acid from forming.

"It's not exactly a simple question," he said.

But, he says, the environmental impacts of the Ok Tedi mine have been consistently underestimated since the mine began and he thinks the company's assessment is "optimistic".

Both Dr Mudd and Dr Campbell say the water level in the river is currently artificially high because of the accumulated sediment in the river bed.

"In the future this area will dry out maybe in 10 years, maybe in 100, maybe in 1,000 - but whenever that happens this will cause a major problem for the floodplain and those that depend on it," Dr Campbell said.

And both say climate change predictions for tropical areas such as this are highly uncertain, which means flash floods could occur and threaten the storage pits that are being built.

"All it takes is one big flood to go through and break some of channels, that wasn't predicted, and you get the channel changing course," Dr Mudd said.

Future vigilance?

Perth-based environmental scientist Dr Alan Tingay agrees the new waste dumps could be of concern.

"The last I heard was the mine was going to close down in 2013," said Dr Tingay, who until last year was an independent adviser to a process designed to compensate communities which have been affected by other impacts of the mine.

"These dumps will be there forever and their integrity I think can not be guaranteed.

"Basically [the company is] trying a large scale environmental experiment in an extreme rainfall zone where there is significant potential, in my view, for erosion."

Dr Tingay is particularly concerned about the challenge in revegetating the waste dumps and ensuring their protection from erosion in the long term.

"There are no provisions that I'm aware of for the ongoing maintenance of those dumps or for repair of those dumps should it be required," he said.

Environmental concerns have dogged Ok Tedi mine for many years. The mine was originally 52 per cent owned by BHP Billiton, which sold its stake in 2002 after concerns over environmental damage caused by the mine.